Brazing is a process of joining or uniting an assembly of two or more materials into one structure. Brazing is achieved by heating the materials to a temperature below the solidus temperature of both the materials in the presence of a braze and an optional flux. A brazed body is a metal to metal bond joined by braze. In some instances, the flux is added to reduce the surface of one or both of the metals prior to the braze, creating a surface more agreeable to wetting and therefore achieving a better bond.
Some known fluxes, such as those containing non-metallic ionic salts such as borides and fluorides, work by dissolving oxides at low temperatures. For example, a fluoride flux commercially available under the trade name HANDY FLUX (commercially available from Lucas Milhaupt, Inc., Cudahy, Wis.), contains fluorides that begin to dissolve oxides at 315° C. and remain active from 590° C. to 870° C. These fluxes can cause undesirable results in some circumstances, because this type of flux can leave residual flux and reaction products of the flux within the brazed body that eventually contribute to corrosion spots in the brazed body. Additionally, these fluxes may deplete the surface of a metal of non-corrodible materials, leaving only corrodible materials in its place, for example leaving an iron-rich surface on stainless steel. Minimizing the amount of this type of flux assists in reducing the amount of corrosion that may eventually appear in a brazed body.
A preferred environment for brazing is a vacuum furnace because it minimizes the oxygen in the atmosphere available to attack the hot metals. Fluxes containing borides and fluorides are too volatile to be used in a vacuum furnace, and even a vacuum furnace is unable to stop all metals from oxidizing on their surface. An example of such a metal is stainless steel. An oxide of chromium on the surface of stainless steel inhibits the wetting of stainless steel parts, even in a vacuum furnace. One known solution is to plate a thin layer of nickel over the stainless steel. The braze will wet the nickel surface and metal diffusion will increase the bond between the plated nickel and the base of stainless steel. While plating the stainless steel with nickel works, it requires an expensive plating step and introduces quality assurance problems in making sure the plated nickel meets the requirements for brazing such as uniform coverage and adhesion.
Another known solution is mixing hydrogen into the vacuum furnace to react with any oxygen in the furnace, thereby delivering an oxygen-free environment as described in Brazing of Stainless Steel, Paul F. Stratton, Heat Treating Progress, p.p. H14-H16 (August 2000). However, in actual practice, an oxygen free environment is nearly impossible to achieve. This is especially true for certain metals (e.g. chromium) that oxidize so readily that they would require an extremely dry hydrogen atmosphere.
Therefore, it is desired to have a brazing flux that is non-volatile in the vacuum furnace and still reduces the surface of metals, especially easily oxidized metals. Additionally, it is desirable to have a simple and inexpensive method of brazing metals that are difficult to join.